A characteristic impedance of the air is about 377Ω which can be roughly calculated from a dielectric constant ε0 and a magnetic permeability μ0 in a vacuum state. With this regard, a characteristic impedance used in a high-frequency circuit is 50 Ω-series. A resonant antenna system is used which is represented by a dipolar antenna in which an impedance ratio of these impedances is large by six times (a voltage standing wave ratio VSWR=7.5), and an antenna length of λ/2 is used for radiation from the 50 Ω-series transmission line to the air. A patch element frequently employed in the millimeter-wave band is also an antenna of which the size of conductor becomes a length of λ/2 for resonance.
The patch antenna can be fabricated on a conductor surface on a plate board. Therefore, thinning and cost down can be easily achieved. An opening area having a gain of 20 dBi or more in the millimeter-wave band (76 GHz) is necessarily formed in a size of about 5 cm. There are various types of power feeding circuits such as a tournament type, a branch type (PTL 1), a series fed type (PTL 2) in order to efficiently radiate the millimeter wave from the patch element disposed in the entire area. The power feeding circuit is designed using a λ/4 matching device many times in order to distribute power from one power feeding point to the patch element while achieving an impedance matching. Therefore, in a patch array antenna which uses the patch element and the λ/4 matching device, a frequency bandwidth for a good reflection characteristic (hereinafter, abbreviated as use bandwidth) is about 5% of a carrier frequency.
With this regard, a horn antenna using a waveguide pipe has a bypass characteristic at a frequency higher than a cut-off frequency, and thus has a wide bandwidth. However, a characteristic impedance of the waveguide pipe is 300Ω high, and a reflection coefficient with respect to the connection with a 50 Ω-series transmission path is large. As a countermeasure, a millimeter-band transceiver (PTL 4) is considered in which a plurality of microstrip-waveguide pipe converters (PTL 3) and a plurality of λ/4 matching devices are used through a resonator of the patch element. The patch element and the λ/4 matching device used in these structures are provided in a metal conductor of a multilayered board. In order to achieve a wide bandwidth even in such a conversion structure, there are employed a method in which a plurality of patch elements having different resonant frequencies and a method of disposing a plurality of λ/4 matching devices in a stepped manner so as to reduce a variation of a characteristic impedance. The number of layers of the multilayered board and a processing cost are inevitably increased.
In the horn antenna, there is an antenna which is provided with a ridge structure in a signal input portion and reduced in impedance. A waveguide pipe structure has a cut-off frequency, and has an electrical characteristic such as a bypass characteristic of a wide bandwidth. In the horn antenna with ridge fabricated in general, the input portion of the transmission line is configured in a coaxial structure. Even though there is a parasitic component generated from a difference between the coaxial structure and the metal conductor of the ridge structure, a phase variation is less in a microwave band in which the wavelength is 1 cm or more, and thus there is less influence on the 50 Ω-series transmission path.